A process for forming a single crystal silicon ingot from varying sized pieces of polycrystalline silicon source material according to the Czochralski method. More specifically, the present invention relates to a method of stacking polycrystalline silicon pieces into a crucible for single crystal silicon production by the Czochralski method.
Most semiconductor chips used in electronic devices are fabricated from single crystal silicon prepared by the Czochralski process. In this process, a single crystal silicon ingot is produced by melting polycrystalline silicon source material stacked within a quartz crucible, stabilizing the crucible and source melt at an equilibrium temperature, dipping a seed crystal into the source melt, withdrawing the seed crystal as the source melt crystallizes on the seed to form a single crystal ingot, and pulling the ingot as it grows. Melting occurs at a temperature of 1420xc2x0 C. in an inert gas environment at low pressure. The crucible is continually rotated about a generally vertical axis as the crystal grows. The rate at which the ingot is pulled from the source melt is selected to form an ingot having a desired diameter. The polycrystalline silicon melted to form the molten silicon is typically irregularly shaped chunks prepared by the Siemens process and subsequently broken into suitably sized pieces.
The initial charging of chunk type polycrystalline silicon placed into the crucible and the melting thereof can introduce undesirable impurities and subsequently contributes to defects in the single crystal ingot. For example, when a crucible is initially charged with chunk polycrystalline silicon the edges of the chunks under the load of a full charge can scratch and gouge the crucible wall resulting in a damaged crucible and in particles of crucible floating on or being suspended in the silicon melt. These impurities significantly increase the likelihood of dislocations forming within the single crystal, and decrease the dislocation-free single crystal production yields and throughput. Additionally, initial loadings of 100% irregularly shaped chunk polycrystalline silicon limits the volume of material which can be charged due to the poor packing densities of such chunk materials. The volume limitation directly impact single crystal throughput in the Czochralski-type process.
Holder et al., U.S. Pat. No. 6,284,040, disclose a process for forming a single crystal silicon ingot from varying sized chunks of polycrystalline silicon according to the Czochralski method which includes classifying each chunk of silicon by size and placing the chunks according to size classification within at least three regions of the crucible.
Holder et al., U.S. Pat. No. 5,588,993, disclose a process for forming a single crystal silicon ingot by the Czochralski method. In the preferred method, chunks of polycrystalline silicon are first loaded into a crucible forming a central depression therein, a partial melt is formed, and particulate polycrystalline silicon is fed into the unmelted region of the central depression.
Holder, U.S. Pat. No. 5,919,303, describes a process for preparing a silicon melt from a polysilicon charge by the Czochralski method where a crucible is loaded with chunk polysilicon to form a charge having a bowl-like shape, where initially the load generally slopes radially upwardly and outwardly from the centerline toward the sidewall formation to an apex and then slopes generally downwardly and outwardly from the apex to the sidewall formation. The bowl-shaped chunk polysilicon charge is heated to form a partially melted charge, and granular polysilicon is fed onto the partially melted charge to form a mixed charge of chunk and granular polysilicon.
The present method of stacking the polycrystalline silicon pieces in the crucible allows for a denser packing of silicon in the crucible, can be accomplished in a quicker time then conventional packing methods, and has the potential for less damage to the crucible bottom, when compared to standard packing methods using a size assortment of irregular shaped silicon pieces.
A process for forming a single crystal silicon ingot from varying sized pieces of polycrystalline silicon source material according to the Czochralski method is disclosed. In one embodiment of the invention, the process comprises placing into a crucible on the bottom a generally polygonal-shaped concentric array of rod-shaped polycrystalline silicon pieces having obliquely cut ends. In an alternative embodiment of the invention, the process comprises placing into a crucible on the bottom a generally parallel array of rod-shaped polycrystalline silicon pieces having obliquely cut ends.